1. Field of the Invention
The invention relates to a belt press unit, which preferably serves as a wet press of a paper machine. More particularly, the invention relates to the support by a support beam of the press belt of the belt press unit, particularly with a press belt which may be inflated.
A belt press unit in a paper machine is a pressing device which has a so-called "extended press nip." This means that the press nip is formed between a rotating surface, preferably the cylindrical surface of an ordinary press roll (or "back roll"), on the one hand, and an elastic tubular press belt, on the other hand. The press belt is adapted to be pressed against the back roll by means of a pressing device (for instance, a press shoe). In this way, the press nip that is produced is relatively long in the circumferential direction so that pressure is exerted on the web of paper (which passes through the press nip together with a felt belt) over a greater length of the press belt than in a traditional roll press cnsisting of two opposing rolls.
2. Description of the Prior Art
Various prior art references disclose extended nip presses.
German Patent Application DE-OS No. 31 26 492 discloses a press unit with a box-shaped supporting body around which the tubular press belt travels. Outside the press zone, the belt travels at a large distance away from the supporting body. The ends of the press belt are fastened to tensioning disks which can rotate on the journal pins of the supporting body. One disadvantage of this construction is that no guide elements for the press belt are present outside the press zone, as seen in cross-section. There is a danger that the press belt will not more smoothly, particlarly at high operating speeds.
German Patent Applicaton DE-OS No. 31 02 526 discloses belt press units having a substantially roll-shaped supporting member. In accordance with FIG. 1, the press belt rotates predominantly at a slight distance from the outer surface of the supporting body. According to FIGS. 4 and 5, the press belt slides over the supporting body. In this case, high frictional heat is produced. A large number of lubricating chambers are distributed over the circumference of the outer clindrical surface of the supporting body for the feeding of a lubricating liquid. Outside the press zone, a slide shoe is provided in the supporting body. This shoe cooperates with a drive roll and can be used also for retightening the press belt.
U.S. Pat. No. 4,287,021 shows a press belt which passes predominantly around the supporting body at a slight distance from it. The supporting body has an "apple"-shaped cross-section which is difficult to produce. Before entrance into the press zone, the press belt is guided by an outer curved guide surface in order to avoid fluttering of the press belt.
German Patent Application DE-OS No. 31 26 492, noted above, and U.S. Pat. No. 3,269,893 and German Patent Application DE-OS No. 19 23 784 show that the tubular press belt can form a closed inner space together with two side tensioning disks. This inner space can be inflated by compressed air, whereby one speaks of an "inflatable tube roll". Such a closed inner space is also provided in above noted DE-OS No. 31 02 526, but not in U.S. Pat. No. 4,287,021 since in the latter case, the cross-section of the supporting body differs too much from a circular shape. In the latter case, the inner space surrounded by the press belt is open toward the outside on both ends so that there is an danger of dirtying the machine by emerging lubricating oil.
The construction in German Patent Application No. 19 23 784, FIGS. 11 and 12, guides the press belt in the region outside the press zone by guide rolls which are mounted on the supporting body. One disadvantage of this construction is that it can be used, at most, for very narrow width paper machines because the guide rolls, which must be arranged within the tubular press belt, can only be of small diameter and thus are deflected (at least under their own weight).
The invention proceeds from the known construction including a tubular, laterally closed press belt and in which, as seen in cross-section, the press belt is guided outside the press zone by a guide device. This design has been generally known for more than twelve years. It is also known that substantially higher solid cntents can be obtained in the web of paper to be drained when using a wet press with an extended press nip at the end of the press end than when using traditional roll presses and that a considerable saving of energy can be obtained upon the subsequent thermal drying of the web of paper. Nevertheless, no one has, to the best of the applicant's knowledge, employed a belt press unit according to any of the above prior art references in the press end of an industrially employed paper machine. Only in the type of construction according to German Patent Application DE-OS No. 30 40 891, equivalent to U.S. Pat. No. 4,272,317 are wet presses with extended press nip presently used. In that case, the press belt travels over several guide rolls. Some of those rolls are mounted in swing levers for the tensioning of the press belt. The structural expense of this is greater than when using a tubular press belt. Furthermore, there is the danger of dirtying from lubricant which emerges laterally from the press-belt loop.
The difficulities which have stood in the way of the use of a belt press unit with a tubular press belt are caused primarily by the numerous requirements for a suitable design.
1. The press belt must not twist or form wrinkles. In other words, the points of an imaginary line extending transverse to the direction of rotation of the press belt must all move with exactly the same speed.
2. The amount of drive energy required for rotation of the press belt should be as small as possible. This also applies to the start-up of the belt press unit, i.e., the starting torque should be as small as possible. Usually, a separate drive is not provided for the press belt. Instead, it is carried along by the felt belt on which the paper web is dewatered.
3. The life of the press belt and of the press shoe should be as long as possible. For this, the heat produced must be effectively removed.
4. The above requirements apply also for the dimensions (work width up to 10 m) and travel speeds (on the order of 1000 m/min.) which are customary in modern paper machines.